1. Technical Field
The present invention relates to semiconductor processing, and more particularly to fabrication processes and device structures using wafer transfer to form III-nitride electronic devices.
2. Description of the Related Art
Group III nitride materials are a unique group of semiconductor materials, which can be used in a wide variety of applications including, for example, optoelectronics, photovoltaics and lighting. Group III nitride materials are composed of nitrogen and at least one element from Group III, i.e., aluminum (Al), gallium (Ga) and indium (In). Illustrative examples of some common gallium nitrides are GaN, GaAlN, and GaAlInN. By changing the composition of Al, Ga and/or In within a Group III nitride material, the Group III nitride material can be tuned along the electromagnetic spectrum; mainly from 210 nm to 1770 nm. This spectrum includes a visible light emitting diode (LED), which is more than a 10 billion dollar industry with a forecasted double digit yearly growth rate. This continuous growth in LED demand enables the infrastructural build-up for the growth and fabrication of Group III nitride based semiconductor devices.
Light emitting diodes (LEDs) are energy efficient replacements to conventional light bulbs. LEDs are typically formed on a substrate and packaged in chips. The cost of conventional substrates (e.g., sapphire) is high, which increases the price of an LED chip. Such substrates are not reusable. LEDs include a front side contact and a back side contact formed on a same side of a substrate. Nonconductive substrates prevent back contact allocation, which leads to both contacts being formed on one side of the LED. This side-to-side contact allocation leads to current crowding effects degrading the carrier injection.
In addition, thermal conductivity of conventional sapphire substrates is about 42 W/m-k. Sapphire substrates provide marginal thermal management and produce higher junction temperatures. This leads to degradation of device characteristics and lifetime.
For other substrates, e.g., InGaN with an index of refraction (nGaN) of 2.5, and critical angle (θc) of about 23°, the light extraction efficiency is expected to be only 4%, assuming that one light escape cone is present in the LED structure. This leads to reduced external quantum efficiencies as generated light cannot couple out efficiently.